This invention relates generally to the measurement of fluid flow. More particularly, the present invention is directed towards an in-line parallel proportionally partitioned by-pass metering device and method of measuring fluid flow within a closed conduit system.
Fluid flow measurement is widely practiced and fulfills an array of purposes including energy distribution, custody transfer, regulation, control and research. The measuring unit, i.e. "flowmeter", typically consists of a primary and a secondary device. The primary device is acted upon by the fluid directly, and the secondary device converts the primary device's response to the fluid into an observable quantity. Flowmeters are generally classified into those which measure quantity of fluid flow and those which measure rate.
Because of the wide practice of fluid flow measurement, engineers have a greater choice when specifying a flowmeter than for perhaps any other process measuring, monitoring device. Currently, there are over one hundred types of flowmeters available, and expenditures on flowmeters exceed one billion dollars per year. Thus in choosing a flowmeter, an engineer will typically evaluate: (1) the degree of accuracy and/or precision required, (2) the suitability of the flowmeter to the particular application and conditions, and (3) the cost of the various alternative flowmeters and other limitations such as space requirements, and the like.
The selection of a flowmeter requires an understanding of the flow behavior of fluids. First, a fluid is any matter which undergoes continuous deformation upon being subjected to shearing forces. Viscosity is that property of a fluid by which it offers resistance to deformation or shear. The response of a fluid subjected to shearing forces is "flow". The type of flow, whether laminar, turbulent, cavitational, or some combination, depends upon the fluid viscosity and other parameters of the fluid flow system. Second, a fluid in motion, i.e., flowing, possesses energy. This energy may be displacement or pressure energy, velocity or kinetic energy, potential energy, thermal or internal energy, or some combination of these forms of energy. Flowmeters utilize the energy of a fluid in motion to monitor fluid flow. Therefore, an engineer must also consider the type of flow and the energy of the fluid flow when selecting a flowmeter.
Flowmeters which measure quantity repeatedly measure a fixed volume of fluid. These flowmeters are generally of the reciprocating or rotating piston, nutating disk, or rotary vane type. A limited number of flowmeters are available to measure volume or quantity of fluid flow. There is more selection when choosing a flowmeter to measure rate of fluid flow. These flowmeters generally measure differential pressure, area, velocity, heat area, thermal, or other characteristics of the fluid flow from which the rate of flow may be determined. Examples of types of flowmeters which measure differential pressures include orifice, venturi, flow nozzle, and pitot tube devices. Flowmeters which measure velocity include cup, propeller and turbine type devices. Each of these devices are more or less suitable for a particular application based on conduit size, type of fluid, and required accuracy.
Traditionally, flowmeters have been of the "full bore" design; that is, the flowmeter is of the same size as the conduit in which the fluid is flowing. While this is economical for small conduit sizes, flowmeters for large conduit sizes, e.g., in excess of four inches in diameter, may cost thousands of dollars. Furthermore, the "full bore" flowmeter is typically situated directly in the main conduit line. Thus, when the meter requires service or other maintenance the fluid flow in the main conduit must be stopped which causes operational losses. Still further, "full bore" flowmeters, particularly those in large diameter conduit systems, are exposed to high stresses generated by the fluid flow and potential corrosion or erosion due to fluid exposure. Thus, these meters are typically constructed of heavy duty materials such as cast iron, aluminum, bronze or other similar metals. However, these materials may potentially leach harmful elements, such as lead from bronze, into the fluid flow. This undesirable effect is of particular concern in applications which monitor fluid flow for human or livestock consumption.
In response to the high cost and maintenance of large flowmeters, a class of flowmeters known as "insertion type" flowmeters have become commercially available. Insertion type flowmeters, suitable for applications involving the measurement of flow rate, infer an overall flow rate based on the measurement of fluid velocity at particular locations within the conduit. These types of meters are typically utilized in conjunction with larger conduit sizes (i.e., greater than six inches) and where repeatability, not accuracy, is the prime requirement. The accuracy of insertion type flowmeters is limited by a number of factors including accuracy of the primary and secondary meter elements, the position of the primary meter element when inserted into the fluid stream, the velocity profile of the fluid stream, and variation or uncertainty of the inside diameter of the conduit. A further source of inaccuracy with some types of insertion flowmeters is that they sample the fluid stream at a right angle to the fluid path thus causing an abrupt change in the fluid direction. This abrupt change in fluid direction can cause distortions of the fluid flow which further reduces accuracy. At best, a typical insertion type flowmeter will have an accuracy less than 95%. This compares adversely to a full bore type flowmeter which can have an accuracy exceeding 99%. Such a difference in accuracy can be quite significant. In an exemplary application where fluid flow averages 100 gallons per minute, at the end of one year the potential error in quantity of fluid measured by an insertion type flowmeter compared to a full bore type flowmeter may be over 2 million gallons.
An example of an insertion type flowmeter is described in U.S. Pat. Nos. 3,581,565 and 3,803,921 to Dieterich. The Dieterich device is a multi-port slidable flow measuring device of the differential pressure class. Specifically, it is a modified pitot tube device incorporating an interpolating tube for averaging fluid samples. In this arrangement the sampling tube is inserted across the fluid path thus sampling the fluid stream at a right angle. As discussed, sampling of the fluid at right angles to the fluid flow is undesirable as this causes an abrupt change in the fluid flow direction thereby causing distortions (such as eddy currents) of the fluid flow and reducing accuracy. The Dieterich device has been used for low flow gas measurements in stacks and flues where the sample is caused to route through a bypass prior to reentering the fluid stream. This bypass, as suggested, contains a costly auxiliary measuring sensor such as a hot wire anemometer device to monitor the gas flow.
The Dieterich device hence is distinguishable from the present invention in that it incorporates a multi-port tube inserted vertically across the fluid path, uses an auxiliary measuring device, is movable within the fluid path, and is adaptable mainly to conduit sizes in excess of three inches. Such a device further requires relatively high mechanical dexterity and know-how to install and operate and, as described, has an undesirably high cost associated with the flow measuring sensor.
It is another object of the present invention to provide an apparatus for measuring total fluid flow within a closed conduit system by a reduced size flowmeter. Accordingly, it is an object of the present invention to provide an apparatus for measuring total fluid flow within a closed conduit system with at least the same accuracy and reproducibility as a full bore flowmeter yet without the cost attendant therewith.
It is still another object of the present invention to provide a method of accurately measuring the rate or quantity of fluid flow within a closed conduit system by measuring the flow of a proportional amount of the fluid flow.
It is a further object of the present invention to provide an apparatus suitable for use with flowmeters constructed of alternative materials such as plastics, ceramics, and metal alloys.
It is still another object of the present invention to provide a fluid flow measuring device readily adaptable for use with most commercially available flowmeters.